Hot air for cooking food in baker's ovens can be produced by burning a fuel in a burner and transferring the heat in the exhaust gases via a cross-flow tube heat exchanger to cooking air without the cooking air being contaminated by the exhaust gases. The heated cooking air is transported into the cooking chamber of the oven. Conventional baker's ovens use S-shaped ducts arranged inside a heating chamber to transport the hot exhaust air from the burner to a chimney, with the substantially horizontal bottom section of the S connected to the outlet of the burner, the substantially horizontal intermediate section of the S containing heat exchanger pipes to heat up the cooking air and the substantially horizontal upper section being connected to a chimney. The heat exchanger pipes are normally in the shape of tubes with circular transverse cross-section or rectangular with rounded short sides and are made of a material with a high coefficient of heat transfer. The exhaust gases pass though the inside (known as the “hot side”) of the heat exchanger heat transfer tubes and the cooking air is forced to flow around the outside of the tubes (known as the “cold side” of the heat exchanger) in a direction which is substantially perpendicular to the flow inside the tubes. Normally the cooking air enters the heating chamber at the bottom of the chamber where the exhaust gas is at its lowest temperature and circulates up past the heat exchanger tubes and burner exhaust tube where the temperature is highest before being transported into the cooking chamber of an oven. This flow of air is driven by a fan. The amount of heat transferred from the exhaust gases to the cooking air is influenced by the surface area of the heat exchanger surfaces in contact with the exhaust gases and cooking air, the heat transfer coefficient of the heat exchanger (which is influenced, amongst others, by the materials used and their surface properties, and the air flow in and around the heat exchanger) and the temperature difference across the walls of the heat exchanger. While conventional heat exchangers are relatively compact and have good efficiency they still take up a lot of space and are a major contributor to the foot print of a baker's oven. It would be desirable to improve the efficiency of the conventional heat exchangers. This would lead to the advantages of being able to produce baker's ovens with a smaller footprint and/or lower running costs as increased efficiency would lead to less fuel being required to heat the same amount of cooking air. Conventional methods for increasing the heat flow from the exhaust gases to the cooking air in the heat exchanger include changing the shapes of the heat exchanger tubes to increase the surface area exposed to the cooking air or to add more tubes. Such measures may increase the efficiency slightly but at the cost of increased air friction on the cold side which requires the use of a larger fan and increased running costs.
GB-2424265 relates to heat exchange tubes with integral fins. In the background it is mentioned that it is common knowledge to provide a vessel through which the heated fluid is flowing with as large a surface area as possible in order to maximise the transfer of heat. The interior of the tubular body is typically divided into a plurality of longitudinal passages, the dividing walls between the passages serving both to strengthen the tubular body and to increase the surface area over which transmission of heat to or from the fluid flowing through the passages may take place.
US-2003/209344 relates to a heat exchanger comprising tubes, where each of the tubes includes a passageway, which is divided into a plurality of sub-passageways. Each of the sub-passageways is generally rectangular in shape (other geometrical shapes are mentioned) and has a finned interior wall surface.
DE-102005020727 and EP-0359358D3 also relate to heat exchangers provided with tubes having two or several channels.
However, none of the background prior art documents includes any heat exchanger tubes specifically adapted for use in hot air ovens.